To delete a webform:

Navigate to the webform you want to delete

Go to:  Communication area > Webforms / Surveys

This will load a page with several clickable links to the right of the webform name.

Click the "edit" link across from the webform name. Then, scroll to the bottom of the page and click delete.

Do you just want to clear all past results?
Go to:  Communication area > Webforms / Surveys
This will load a page with several clickable links to the right of the webform name. Click the link labeled "Clear". This will clear all previous entries in the webform.